Rapid escalation of energy costs in recent years has spurred efforts to improve efficiency in plant operations. Distillation practices have received great attention, since the process of distillation, the predominant unit operation in the oil refining and chemical industries, consumes prodigious amounts of energy.
One energy-saving procedure is the practice of multiple stage, e.g., two-stage, condensation of the overhead vapors from a distillation column. Multi-stage condensation procedures allow energy savings by permitting the use of higher temperatures at which heat may be removed from the overhead vapor. Such a process is described, for example, in U.S. Pat. No. 3,320,159. In at least one two-stage procedure for condensation of distillation column overhead vapors, the first stage of condensation is operated under conditions to condense just enough liquid to provide reflux for the distillation column, and the second stage of condensation is operated under conditions to provide the top product of the column. For two-stage condensation to be of value in heat recovery, the vapor or top product from the distillation column must be a multi-component mixture, and there must be a reasonable temperature spread between its dew point and bubble point, e.g., 20.degree. F. Most hydrocarbon distillations fit this requirement.
A very common commercial process for which two-stage condensation can be helpful in the separation of gasoline boiling range materials (often 400.degree. to 430.degree. F. ASTM end point) as the top product from a feed containing also kerosene, jet fuel and gas oil components. The present process is particularly useful for separating C.sub.5 to +450.degree. F. boiling range naphtha into low boiling and higher boiling fractions, e.g., deisohexanizing naphtha.
The present invention is directed to a process to improve the distillation column control in order to minimize the consumption of energy. It takes less energy to effect a separation by distillation if the multicomponent compositions of the product streams are held constant compared to the energy required to accomplish the same separation where the product compositions average out to the desired value, but fluctuate widely above and below the desired value during the course of the distillation.
Most distillation columns have an accumulator vessel for condensed overhead vapor to serve as a surge volume for reflux and top products. The composition inertia of the liquid inventory in the accumulator results in lag time which dampens swings in the column overhead vapor composition. This lag time detrimentally affects the responsiveness of the distillation control system to minimize fluctuation in the composition of the distillate product. That is, the composition of the accumulator may vary from the composition of the condensed liquid to the accumulator, making sharp control of the distillation column by use of the accumulator liquid much more difficult. Accordingly, a need has existed for a process which would provide the advantages of multistage condensation heat recovery and/or lower condenser cost and of sharpened control of the distillation process without sacrificing the surge volume required for smoothing flows and protecting pumps employed in the process. The present invention satisfies that need.
A number of patents have recently issued concerning improvements in the condensation process employing an accumulation zone sectioned by a barrier which provides limited flow of liquid between the sections. These include U.S. Pat. Nos. 4,235,706; 4,484,983; 4,484,984; 4,484,985 and 4,484,986, incorporated herein by reference.